This invention relates to moisture sensors for detecting moisture and other particles such as rainfall, dew, dust and the like on vehicle windows for controlling vehicular accessories such as windshield wipers and the like. More particularly, the invention concerns an optical sensing apparatus and assembly for mounting the sensor on one side of a window panel for detecting moisture on the opposite side of a window panel while compensating for various environmental factors.
Various types of moisture sensing apparatus are known for use on vehicles to control accessories such as windshield wipers. These include resistance, capacitance, microwave, vibration and optical sensing devices. Many of these devices require application on the exterior of the vehicle. For example, resistance or capacitance measuring sensors typically require coatings or other structure on the exterior of a window or vehicle for physically contacting rainfall or moisture. These sensors require special production methods in manufacturing the window and significantly raise the window cost. Likewise, vibration sensors must be placed on the outside of the vehicle where rain, sleet, hail or the like can be sensed to create vibration for activation of the desired wiper or other accessory.
One moisture sensing device found particularly useful is an optical sensing apparatus employing an infrared emitter and detector which senses infrared energy emitted from inside the windshield or window and reflected back at decreased levels when moisture is present on the outside of the windshield to a detector also mounted on the interior of the vehicle. Such a sensor, which can be adapted to virtually any windshield without the need for special glass processing, is shown and described in U.S. patent application Ser. No. 073,496, filed July 15, 1987 by Peter A. Hochstein. That moisture sensor includes an emitter which emits pulses of radiant infrared energy toward the inside surface of a window at an incident angle. The radiant energy is refracted into the window at the inside air/window surface and reflected off the air/outside window surface back through the window where it is detected by a detector positioned along a reflection angle equal to the incident angle. The emitter and detector are spaced by a distance D as determined by the following formula where T is the thickness of the glass, n is the index of refraction for the glass, and .theta. is the angle of incidence and reflection of the infrared energy: EQU D=2T cos .theta. (n.sup.2 -cos.sup.2 .theta.).sup.1/2
While this moisture sensing device has been found to be economical and reliable within a small predetermined temperature range, experimentation with the device has indicated that operation is inadequate over the typical automotive operating temperature range of -40.degree. C. to +85.degree. C. Indeed, it has been found that varying temperatures significantly affect the amount of infrared energy emitted thereby significantly changing the operational response of the moisture sensor incorporating such emitters.
In addition, tests have determined that typical infrared emitting elements used in the moisture sensor device of the above type also degrade in the amount of energy emitted over their lifetime. This also causes reduced operational effectiveness as infrared emission levels fall with age.
Further, it has been found that significant amounts of ambient light or infrared energy present in the environment in which the vehicle including the moisture sensor is operated can significantly affect and hinder operation of the accessory control using the device. For example, when the device is mounted on the front windshield of a vehicle, reflected light from the vehicle hood, or from snow which is present on the vehicle or on surrounding land or adjacent the road on which the vehicle travels can saturate the detecting system and prevent proper operation. Similarly, operation of a vehicle on inclines, at sunset, in the vicinity of large numbers of other vehicles whose head or tail lights are illuminated, or around street lights may cause premature operation of the unit and operation of the windshield wipers when no moisture is present.
Related problems regarding use of the moisture sensing units include maintaining proper geometrical relationship between the emitter and detector on the window or windshield in keeping with the above formula while allowing ease in removal and replacement of the unit for repairs or to allow replacement of the window or windshield if cracked or broken. Also, since the moisture sensing unit requires electrical power, an aesthetically pleasing housing was desired which could position the sensor within the sweep area of the windshield wipers while shielding and covering required electrical wiring.